1. Field of the Invention
This invention relates to optical fibre customer lead in (CLI), that is to say to the way in which optical fibres (such as telecommunications optical fibres ) are led into customers' premises.
2. Related Art
In the United Kingdom, the telecommunications network includes a trunk network which is substantially completely constituted by optical fibre, and a local access network which is substantially completely constituted by copper pairs. Eventually, it is expected that the entire network, including the access network, will be constituted by fibre.
The ultimate goal is a fixed, resilient, transparent telecommunications infrastructure for the optical access network, with capacity for all foreseeable service requirements. One way of achieving this would be to create a fully-managed fibre network in the form of a thin, widespread overlay for the whole access topography, as this would exploit the existing valuable access network infrastructure. Such a network could be equipped as needs arise, and thereby could result in capital expenditure savings, since the major part of the investment will be the provision of terminal equipment on a `just in time` basis. It should also enable the rapid provision of extra lines to new or existing customers, and flexible provision or reconfiguration of telephony services.
In order to be completely future proof, the network should be single mode optical fibre, with no bandwidth limiting active electronics within the infrastructure. Consequently, only passive optical networks (PONs) which can offer this total transparency and complete freedom for upgrade, should be considered.
The most common passive optical network is the simplex single star, with point-to-point fibre for each transmit and receive path, from the exchange head end (HE) to the customer network terminating equipment (NTE). Thus, each customer is serviced by a pair of optical fibres. This network design has been used throughout the world and meets all the access criteria. It involves high fibre count cables, and unique electro-optic provision at HE and NTE for each customer.
In order to achieve the goal of providing a fixed, resilient, transparent telecommunications optical fibre access network, it will be important to minimise the installation costs at each part of the network. The present invention is concerned with minimising the cost of installation of fibre from a customer's premises to the nearest network node, and in particular to a cost-effective way of getting fibre into a customer's premises. In this connection, it should be noted that the preferred way of installing fibre is by the well known fibre blowing process (see EP 108590). In this process, fibre units (usually a two-fibre unit for residential premises) is blown through a small diameter (5 mm) polyethylene tube.
The main problem to be overcome in feeding such a tube into a customer's premises arises from the fact that the tube must be fed along the outer wall of the premises and then turned through 90.degree., so as to pass through a hole formed in the wall, without giving rise to optical loss in the blown-in fibre when it is live.
A known CLI apparatus is constituted by a pair of CLI units, one for fixing to the external surface of a customer's premises wall and one for fixing to the internal surface of that wall. Each of these units is fairly large, as it accommodates a complete loop of tube (fibre), the loop having a radius greater than the minimum bend radius (100 mm) for this type of tube. The hole drilled through the wall between the two CLI units has a fairly large diameter so as to accommodate a bend limiting conduit through which the tube (fibre) passes between the two units. Not only are these CLI units relatively large and require the drilling of a relatively large diameter hole, but they are also expensive and time-consuming to fit. Presently, they are used for business premises CLI, where their cost can be justified by the number of lines normally installed for such premises. For residential customers, however, which usually have only one line, these units are far too expensive. In this connection, it should be noted that there are 14 million residential premises in the United Kingdom.
Another factor which increases the cost of CLI apparatus, is the requirement to provide a gas seal. Thus, the tube (for the blown fibre unit) passes through the wall of a customer's premises, and so would provide a path for the ingress of gases such as natural gas from British Gas pipes (which may pass closely adjacent to the blown fibre tube) or methane (from any source). Building regulations require that residential premises are protected against ingress of such gases. The known type of CLI unit described above incorporates a gas seal constituted by a resin which is injected into the tube at the bottom of the loop. This resin cannot escape as it is setting, as it is at the low point of the loop. Once set, the resin provides an effective seal against the ingress of gases.